What failure mode would you want, if not off? Having a failed sensor+controller control temperature in any way isn't what I'd want. One could have a backup thermostat as a fallback, but I doubt their average customer wants anything that complicated.
The "fail safe" mode here needs to be "maintain moderate temperatures, neither burning nor freezing". Simple mechanical thermostats do this extremely well.
But they don't have api with which to auto post your daily temperature to tumblr. Or security vulnerabilities.